RPA (Rocket Propulsion Analysis) is the tool to own for liquid fuelled rocket engine design.
Although, having been around for a while, downloading the latest update I thought it may make a good post to talk about for those who have never come across it.
Basically, RPA allows you to design your liquid rocket engine from the ground up, carrying out performance analysis, chamber sizing, chamber cooling analysis to name a few of the features.
It is very easy to use and allows quick design and iteration of your rocket engine and can also export a .dxf file of the inner engine geometry for further CAD design.
It can be downloaded as a trial in the lite edition but I would highly recommend buying a full version, it will be an investment you would not regret.